Combustion devices in chemical processing and petrochemical production and refining operations are a major source of NOx emissions. Adiabatic flame temperature reduction is one method of reducing NOx emissions. Efforts to reduce flame temperature, such as increasing the air/fuel ratio and introducing flame diluents to the flame can lead to unstable flames, flame extinction, or flame blow-out that can create potentially dangerous operating conditions (e.g., flooding of the combustion device with unspent fuel).
Flame stability or instability sensors have been developed, and their use in combustion system control systems has been proposed. Increasing the air flow to provide fuel lean operation of the combustion device has also been proposed. However, exclusive reliance on air to reduce flame temperature presents its own set of challenges. For example, air contains a significant amount of oxygen, which is an oxidizing agent. If fuel is introduced to the air stream at an inappropriate location, this can create conditions for flame instability and/or a “flame out” to occur. In unstable and/or “flame out” conditions, the flame is either partly or fully extinguished such that flammable gas enters the furnace, potentially resulting in an explosion. This scenario is applicable to all fired heaters regardless of the process function or any scheme being employed to reduce NOx emissions.
There remains a need to achieve NOx reduction in combustion devices while maintaining a stable flame and safe operating conditions for the combustion device.